A typical turbofan forced mixer consists of a number of mixer lobes that alternately extend radially outwards (crests) and inwards (valleys) to create the geometry necessary for forced mixing. In the current designs of turbofan mixers, the transition from annular gaspath to the forced mixer is designed such that the curve that forms the crest line and the curve that forms the valley line depart from the annular gas path at the same axial location, defined as the cross-over point. Applicant has found that this may lead to diffusion problems.